Anne Arundel Police Department's acting chief William Krampf said: "It's going to be quite a while determining occurred, why it occurred and how it occurred.

"We do have fatalities and we do have serious injuries."

"We also have many, many witnesses being interviewed."

The US President Donald Trump tweeted, offering his "thoughts and prayers (to the) victims and their families".

Prior to departing Wisconsin, I was briefed on the shooting at Capital Gazette in Annapolis, Maryland. My thoughts and prayers are with the victims and their families. Thank you to all of the First Responders who are currently on the scene.